A challenge facing the production of two-dimensional sheet-like materials such as graphene is their strong tendency to aggregate due to the flexibility of the individual sheets and their strong van der Waals attraction to one another. To make matters worse, since these materials typically experience compressive stresses during shaping and manufacturing processes such as drying, calendaring, and pelletizing, their tendency to aggregate is often reinforced by processing. Once aggregated, the technical benefits of such sheet-like materials tend to be diminished. Accessible surface area is reduced, and channels for liquid and gas perfusion between sheet-like materials are concealed or eliminated.
Aggregation of graphene into stacked graphene, for example, remains a challenge when forming graphene via the exfoliation of graphite. One attempt to solve this issue has been to expose the graphene sheets to a surfactant or a solvent that acts to stabilize the graphene sheets and reduce their interlayer interactions. For example, ultrasonic cleavage and chemical exfoliation of graphite frequently entails the use of a surfactant that forms stabilizing layers on each side of the graphene sheets. Nevertheless, once the dispersions are dried, the graphene sheets inevitably begin to aggregate, and making them re-disperse thereafter is extremely difficult. As a result, the adoption of conventional powder processing techniques with graphene synthesized by exfoliation typically yields materials with compromised performance.
An alternative strategy directed at alleviating the interlayer interactions of graphene involves engineering the morphologies of the sheets to form structures that are resistant to the negative effects of aggregation, although this methodology is not admitted as prior art by its inclusion in this Background Section. For example, crumpled graphene balls stabilized by locally folded ridges have been synthesized via evaporating aerosol droplets of graphene oxide (GO). In so doing, the GO sheets were dispersed in water or organic solvents and then rapidly dried, which caused the sheets to deform into highly wrinkled structures as a result of evaporation-induced capillary flow. To restore the conductivity of the structures, the GO was then thermally reduced back to graphene. Unfortunately, the reduction of GO into graphene is almost always incomplete and results in a high degree of structural disorder. Thus, here again, the ultimate product is likely to be compromised.
For the foregoing reasons, there is a need for alternative methods of forming graphene structures with morphologies that are resistant to the negative effects of aggregation and compaction, and are thereby well suited for applications such as reinforced composites, energy storage devices, and sensors.